1. Field of the Invention
Embodiments of the invention relate generally to circuit overcurrent protection. More specifically, at least one embodiment relates to a system and method for isolating circuit protective devices.
2. Discussion of Related Art
In general, overcurrent protection for electrical circuits is provided by circuit breakers, fuses, or a combination of the two. These protective devices are selected and applied according to their current ratings to protect electrical circuits including electrical wires and cables as well as electrical appliances, motors, transformers and other electrical loads. Often an electrical system includes a main circuit breaker or main fuse that supplies a plurality of branch circuits that may each include a separate protective device.
As used herein, the term “circuit protective device” refers to a device that provides overcurrent protection including overcurrent sensing and circuit isolation in response to one or more pre-determined overcurrent conditions.
Generally, a fuse includes a metal wire or strip that will melt when heated by a predetermined amount of electrical current. The fuse rating or nominal current rating is the amount of continuous current that the fuse can carry without having the fuse element melt and open the circuit i.e., without having the fuse “blow”. Accordingly, a 20 amp fuse can carry 20 amps on a continuous basis without opening the circuit due to over-temperature of the element in the fuse. The amount of time that the fuse will carry an overcurrent decreases as the magnitude of the current increases.
Circuit breakers are also designed to open in response to an overcurrent. Because circuit breakers are designed to be reset and closed following an overcurrent trip, they generally include mechanical or electro-mechanical operating mechanisms. Accordingly, the overcurrent protection may include a thermal element that deflects in response to an overcurrent (e.g., a residential molded case circuit breaker), and/or electronic current sensing and electronic trip units (e.g., commercial/industrial circuit breakers).
Overcurrent conditions can result from overloads and short circuits. Both circuit breakers and fuses may experience nuisance operations in which they respond to low-level overloads by opening the circuit and disconnecting the associated load. Fuses may be particularly fast acting and many electrical codes and standard-setting bodies require that fuses be employed in specific applications often because of their operating speed. However, the fast action of a fuse in an overcurrent condition can sometimes result in nuisance failures in which a fuse opens on a temporary overload and must be then replaced. Also, many circuit breakers require manual resetting after they open as a result of an overload.
Although many existing approaches to circuit protection provide a coordinated set of protective devices, the current setting of these devices (i.e., the nominal rated current of the devices) is established based on the need to protect the electrical load. As mentioned above, this can include not only operating equipment such as lighting circuitry, appliances and the like, but also the electrical wiring that connects the various elements of the circuit. Accordingly, existing approaches to circuit protection generally do not take into consideration how the thermal characteristics of the fuse may result in nuisance tripping because the design of the circuit protection is focused on the protection of the equipment and wiring that is supplied by the circuitry. In other words the operation of circuit protective devices on low level overloads is tolerated in the interest of protecting the electrical circuit and devices protected by the protection device.
Another problem with existing approaches involves the use of current sensing circuitry (e.g., a current sensor) having an analog output that is supplied to an input of an analog-to-digital converter (“ADC”). In particular, the accuracy and range of the ADC is limited by the size of the ADC (i.e., the number of bits included in the converter) and the selected resolution of the ADC. These limitations reduce the accuracy of current sensing during some overcurrent conditions. Generally, the resolution and range of the ADC are based on the nominal current rating of the circuit with which it is employed. However, the current carried in an electrical circuit can vary widely from at or below a nominal continuous current (for example, 15 or 20 amps in a residential circuit), to overload current levels that may be 2 or 3 times the nominal continuous current and to short circuit currents that may be tens or hundreds of times greater than the nominal continuous current. The above limitations on ADCs employed with current sensing circuitry may result in inaccuracies in current measurements, and accordingly, in inaccuracies in the overcurrent protection employed with the ADCs. In particular, currents above the maximum current that is accurately represented by the ADC can be “clipped.” That is, the digital representation of the current waveform may have the same value for all magnitudes of current above a maximum.
Yet another problem with existing approaches to overcurrent protection schemes is the fact that consideration of thermal loading is often addressed on a circuit-by-circuit basis. That is, where a system includes a plurality of branch circuits, the thermal loading may be evaluated on a branch circuit-by-branch circuit basis and the thermal capabilities of the entire system may not be adequately addressed. For example, where a transfer switch rated for 100 amps of continuous current includes ten branch circuits each with a separate circuit breaker or fuse to protect the branch circuit, the total nominal rated current of the circuit breakers or fuse in aggregate may far exceed the nominal current rating of the transfer switch. In the preceding example, where ten circuits are each rated for 20 amps apiece, a transfer switch that supplies the ten circuits is, in theory, supplying as much as 200 amps of electrical current. Thus the transfer switch may not include a main circuit protective device and the 100-amp rated transfer switch may be overloaded without any operation of a circuit protective device. Alternatively, where a main circuit protected device is included, existing approaches may simultaneously isolate all ten circuits with the main device in an attempt to provide system wide protection. Such an approach may result in the unwanted isolation of critical loads.